Effects of surface states on the kink phenomena in GaAs MESFETs are studied by two-dimensional simulation. It is shown that the kink could arise due to impact ionization of holes and the following hole trapping by the surface states. It is discussed how the surface-related kink phenomena could be weakened, by analyzing a recessed-gate structure and a structure with n+ source region, which are both expected to have less surface-state effects at the source side.